Best? No.
It’s a great album but there have been plenty of sample based Hip Hop albums since then that have stood the test of time and are up there quality wise and thematically.
What Madvillainy did was make it clear that lyricism that rode the line between “backpack” and gangster had a place and MCs who weren’t part of a huge machine with corporate backing could not only succeed, but be great at rap.
On paper, MF DOOM and Madlib should’ve never created a masterpiece, let alone drawn the audience they did. The average rap fan then and now had/have no idea who Lootpack, Likwit Crew, KMD or Monster Island Czars are, but somehow, the duo of Madvillain became well known through dumb luck and quality that went viral at a time when viral was just becoming a thing.
A masterpiece of an album, but I don’t know if it’s the best in the underground, sample based genre.